You can replant now or wait until fall when the weather is cooler. When you dig, try not to damage too many roots. Expect the majority of the roots to be in the top 12" of soil and radiating out to the tips of the branches. It's okay to sever some of the smaller roots, but try to keep the major rootmass intact. Be sure to water your crape myrtle well after replanting. Good luck with the move.